Various devices for detecting the presence of foreign bodies on the surface of a window are already known. In most cases, advantage is taken of the fact that the transmission of an electromagnetic or elastic wave in the material of the wall is affected by an alteration of the medium in which the wall is located.
Thus, in the case of a windshield of a motor vehicle, the amplitude of an ultrasonic signal reflecting on the surface of an element of the latter may be more or less attenuated as a function of the presence of a foreign body on the surface of said element of the windshield. Detection of this variation in amplitude is thus representative of the presence of foreign bodies on the surface of the windshield and consequently enables a usable control signal to be provided, for example for starting and stopping the operation of a cleaning device.
Patent application EP 0 512 653 proposes an automatically controlled cleaning device, in particular for motor vehicle windshields, which uses this principle and in which an ultrasonic transducer effecting the functions of transmission and reception is directly fixed onto the interior surface of a windshield.
This device operates in a satisfactory fashion when it is used with so-called symmetrical windshields, that is to say comprising two sheets of glass of the same thickness joined together by a connecting layer such as a layer of polyvinyl butadiene (PVB).
When a transducer sets into vibration two sheets of glass joined together by a connecting layer (PVB layer), in order to obtain satisfactory sensitivity to the detection of the presence of foreign bodies on the surface of the sheet of glass remote from the transducer, the vibration amplitude of that sheet of glass must be maximal. This condition is only achieved if the resonant frequency f of the wave emitted by the transducer is substantially equal to one of the resonant frequencies of each of the sheets of glass forming the windshield. Thus, for one sheet i of thickness e.sub.i, f must be close to one of the frequencies: EQU f.sub.i,K =K.v/2e.sub.i, (1)
where f.sub.i,K represent the resonant frequencies of K order (K=1, 2, 3, . . . , n) of sheet i, and v is the speed of the wave emitted by the transducer in the glass in question.
It can be seen therefore that it is easy, in the case of a symmetrical windshield, to select a transducer emitting a signal whose wavelength is adapted to this windshield as the thicknesses of the two sheets of glass forming the windshield are identical. On the other hand, the optimal operating conditions of such a device are no longer so easily fulfilled when the transducer is applied to a so-called asymmetrical windshield, that is to say comprising two sheets of glass of different thicknesses. In such case, to satisfy the above conditions, it is necessary to find a transducer whose frequency f is substantially equal to: EQU f.sub.1 =K.sub.1.v/2e.sub.1 and to f.sub.2 K.sub.2.v/2e.sub.2
where K.sub.1 and K.sub.2 are non zero integers, and e.sub.1 and e.sub.2 are the respective thicknesses of the two sheets of glass, f.sub.1 and f.sub.2 being in this case higher order harmonics of the respective resonant frequencies of the two sheets.
Taking account of the order of size of the thicknesses of the sheets used in a large number of applications (2 to 3 mm), it has been confirmed that these conditions were generally satisfied for high frequencies corresponding to harmonics of the order of 4 to 5 of these resonant frequencies (typically of the order of 5 to 10 Mhz).
The above reasoning disregards, however, the connecting layer which, through the material of which it is formed, absorbs a large part of the ultrasonic waves which pass through it, all the more so when the frequency is high. This connecting layer thus prevents a satisfactory coupling of the two sheets in that it it does not allow optimal sensitivity to be achieved for the detection of the presence of foreign bodies on the surface of the sheet furthest from the transducer.
Another disadvantage of this connecting layer lies in the fact that the speed of propagation of an ultrasonic wave in the material (PVB) of which it is formed varies significantly with the temperature, so that it is impossible to find a frequency f satisfying the equation (1) for the sheets and the connecting layer and for an advantageous range of usage temperatures from a practical point of view.
Another disadvantage of this connecting layer lies in the fact that the acoustic impedance z of the material forming the connecting layer (z.sub.pvb =2.10.sup.6 Kg/m.sup.2.s), is very far from that of the sheets (z.sub.glass =15.10.sup.6 Kg/m.sup.2.s), so that the transmission of the ultrasonic signal at the sheet-connecting layer and connecting layer-sheet interfaces is weak.
It is also to be noted that the use of high frequencies has the disadvantage of requiring the use of thin transducers, which are consequently fragile and delicate to handle.